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'[EE]: Rocket Altimeter Project'
2001\01\18@184723 by Sean H. Breheny

face picon face
Hi all,

A few weeks ago, I asked several questions on here about a rocket altimeter
project that I was working on. Well, I finally built and tested it, and the
preliminary results are at

http://www.people.cornell.edu/pages/shb7/alt.html

Unfortunately, I don't have time right now to put all the stuff I have up
onto the page (like video, code, schematic), so all I have right now are a
text explanation and data graphs. In a week or so I should have the other
stuff up there.

Overall, it was quite successful in that it gave data that makes sense. One
interesting thing is that I observed some strange engine behavior. On the
first test, the engine had about 30% less thrust than it was supposed to,
and on the second test, the engine exploded on the pad (but didn't damage
the altimeter). Do any of you rocket gurus have any idea why? The engines I
used were about 4 or 5 years old, could that be why? Right now I am kicking
myself for putting this much effort into it and then not buying a $6 pack
of new engines :-)

Thanks again for your help,

Sean

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2001\01\18@190217 by David VanHorn

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face
>
>Overall, it was quite successful in that it gave data that makes sense. One
>interesting thing is that I observed some strange engine behavior. On the
>first test, the engine had about 30% less thrust than it was supposed to,
>and on the second test, the engine exploded on the pad (but didn't damage
>the altimeter). Do any of you rocket gurus have any idea why? The engines I
>used were about 4 or 5 years old, could that be why? Right now I am kicking
>myself for putting this much effort into it and then not buying a $6 pack
>of new engines :-)


These weren't Estes "D" engines were they?
They had a real problem with cracked cores (it's basically black powder)
and would explode more often than not.

Try some Aerotech G reloadables. $40 for the casing, about $6 for the
propellant reloads, but 8x the impulse in 29mm diameter!

They also do F engines in the 24mm casing, but the price favours the F ang
G reloads.





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2001\01\18@203925 by Sean H. Breheny

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Hi Dave,

No, they were Estes C6-5's. So, I take it that you think it was just a
coincidence and that age wasn't the reason?(They were never taken out of
the package, just sat sealed up for 5 years). One thing that was strange
was that I heard a slight cracking sound when I pushed the little ignitor
plug in. At the time I thought that it was the ignitor itself and figured
"well, it will either work or not". I don't recall seeing the nozzle crack,
but I wonder if it cracked the propellent somehow? BTW, why does cracked
propellent cause an explosion? It was a very violent explosion, too; did a
lot of damage to the base of my launch stand.

What about the reduced thrust? I looked on Estes's site and couldn't find
the tolerance on thrust figures. 30% seems like a lot.

I hope to try some higher power engines (like F and G) soon, but in this
case, I had already built the rocket and it was intended for C's.

Thanks,

Sean



At 07:00 PM 1/18/01 -0500, you wrote:
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2001\01\18@210919 by Herbert Graf

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> No, they were Estes C6-5's. So, I take it that you think it was just a
> coincidence and that age wasn't the reason?(They were never taken out of
> the package, just sat sealed up for 5 years). One thing that was strange
> was that I heard a slight cracking sound when I pushed the little ignitor
> plug in. At the time I thought that it was the ignitor itself and figured
> "well, it will either work or not". I don't recall seeing the
> nozzle crack,
> but I wonder if it cracked the propellent somehow? BTW, why does cracked
> propellent cause an explosion? It was a very violent explosion, too; did a
> lot of damage to the base of my launch stand.

       From how I understand those motors are constructed I do believe age was the
factor. The bottom of the engine is an epoxy based nozzle. If that nozzle
cracks it will fly apart when the engine ignites. Since the whole
contraption now has the whole diameter of the engine to flare out it burns
much more violently. I have never seen this happen though. I ALSO have
engines which are rather old, I guess I'll be more careful around them in
the future. TTYL

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2001\01\18@224031 by David VanHorn

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At 08:41 PM 1/18/01 -0500, Sean H. Breheny wrote:
>Hi Dave,
>
>No, they were Estes C6-5's. So, I take it that you think it was just a
>coincidence and that age wasn't the reason?(They were never taken out of
>the package, just sat sealed up for 5 years). One thing that was strange
>was that I heard a slight cracking sound when I pushed the little ignitor
>plug in.

That wasn't it. The propellant grain is HARD.  I have seen some cases where
it wasn't cracked, but rather separated from the casing wall.


>At the time I thought that it was the ignitor itself and figured
>"well, it will either work or not". I don't recall seeing the nozzle crack,
>but I wonder if it cracked the propellent somehow? BTW, why does cracked
>propellent cause an explosion? It was a very violent explosion, too; did a
>lot of damage to the base of my launch stand.


If the grain is cracked, then much more surface area is available for
combustion, which raises the temperature and pressure.. Everything's for a
limit.


>What about the reduced thrust? I looked on Estes's site and couldn't find
>the tolerance on thrust figures. 30% seems like a lot.

Their QC seems to have gone to hell in the last few years.


>I hope to try some higher power engines (like F and G) soon, but in this
>case, I had already built the rocket and it was intended for C's.

A G64-10 will certainly give you a boost!

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2001\01\18@224631 by Bob Ammerman

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> and on the second test, the engine exploded on the pad

Wow, was this a standard ESTES engine?

Never heard of that happening before.

Just curious, what would you say the blast radius was (ie: how close could I
have been without getting hurt). I ask because I always have to argue with
people to stay well clear of my model launches and they laugh when they
see/hear the little pfffffffft of the engine.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
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2001\01\18@235113 by David VanHorn

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At 10:43 PM 1/18/01 -0500, Bob Ammerman wrote:
> > and on the second test, the engine exploded on the pad
>
>Wow, was this a standard ESTES engine?
>
>Never heard of that happening before.
>
>Just curious, what would you say the blast radius was (ie: how close could I
>have been without getting hurt). I ask because I always have to argue with
>people to stay well clear of my model launches and they laugh when they
>see/hear the little pfffffffft of the engine.


The D catos that I have seen were not too dangerous. The paper casing is
open at both ends, so it's not so much a detonation as a rapid
overpressure.  The Aerotech reloadables are also designed "fail soft" for
the same reason.

I've never been comfortable with 10' even for the small stuff. I use a 50'
with the small engines, and 100' for everything else.  Control head, with
battery at the pad. Real key lock, dual pushbuttons, and a howler that's
active whenever the key lock is in the armed state.

It lights off copperheads quite reliably, despite the negative press
"crapperheads".


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2001\01\19@005230 by Sean H. Breheny

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Hi Bob,

I've always had respect for them because I used to participate in a
high-school rocket contest and I saw ALL kinds of dangerous things happen
(mostly due to crazy stunts, and not the inherent risk of the engines, I
must admit). Thank God, no one ever was hurt.

Yes, it was a standard Estes C6-5. I've never seen this happen, either.
Only other explosion I remember was an air-burst, but I don't remember what
kind of engine.

I don't have any experience in judging what is a safe distance. It actually
lifted off and went halfway up the rod before bursting. All I can say is
that it sent burning pieces of debris about 5 feet away horizontally, shot
something burning brightly (a piece of the engine?) in an arc which was
about 50 feet high, and managed to tear one leg off the launch pad and
damage another leg, leaving the pad laying horizontally on the ground. It
also spooked some cows which were a couple hundred feet away <VBEG>. Myself
and the other guy were about 15 feet away, and I'm glad we were no closer.

As you can tell, I'm excited to have caught it on video :-) Can't wait to
get it up on my site!

Sean

At 10:43 PM 1/18/01 -0500, you wrote:
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2001\01\19@010739 by Bill Westfield

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   From how I understand those motors are constructed I do believe age
   was the factor. The bottom of the engine is an epoxy based nozzle.

No, it's a pressed clay nozzle.


   If that nozzle cracks it will fly apart when the engine ignites. Since
   the whole contraption now has the whole diameter of the engine to
   flare out it burns much more violently.

No, when the effective nozzle size increases, the burn rate of (any)
propellant DECREASES (with some propellants, enough for the flame to go
out.)  OTOH, you have flames instead of a high-pressure stream of gas,
so it may LOOK more violent.

Pressed black powder motors such as the estes c6-5 are known to be
sensitive to temperature cycling.  The paper casing and the black powder
slug have slight different thermal expansion ratios, and repeated or
extreme temperature changes can cause the propellant to separate enough
from the casing to allow the flame front to burn between them.  When this
happens, the slug ignites all over, overpressurizes the casing, and
(usually) flies out the front of the motor/rocket like a roman candle star.
There's a rule of thumb that goes something like "don't launch at a motor
temperature more than 30F lower than the max temperature the motor has been
stored at."  (Didn't the original message come from someone up at Cornell,
where I imagine it's very cold at the moment?)

As for 30% lower than expected thrust...  I don't think you can accurately
measure thrust with an altimeter - a 30% error in predicted altitude is
likely to be mostly due to errors in calculating the rocket's drag, and/or
weight, and/or air density/etc.  On the other hand, the burn rate of black
powder varies rather dramatically with temperature, so a cold motor MIGHT
actually burn rather slower (and longer) than the same motor launched in
the heat of summer...

BillW

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2001\01\19@013708 by Sean H. Breheny

face picon face
Hi Bill,

At 09:52 PM 1/18/01 -0800, William Chops Westfield wrote:
>     From how I understand those motors are constructed I do believe age
>     was the factor. The bottom of the engine is an epoxy based nozzle.
>
>No, it's a pressed clay nozzle.

That's what it looks like to me, a clay material.




>     If that nozzle cracks it will fly apart when the engine ignites. Since
>     the whole contraption now has the whole diameter of the engine to
>     flare out it burns much more violently.
>
>No, when the effective nozzle size increases, the burn rate of (any)
>propellant DECREASES (with some propellants, enough for the flame to go
>out.)  OTOH, you have flames instead of a high-pressure stream of gas,
>so it may LOOK more violent.

Makes sense, but in my case, it WAS violent, it didn't just look it.



{Quote hidden}

Wow, I think you hit the nail on the head. Those motors had been through
many cycles of temp change, in my basement (where my workshop is and I
frequently heat it up to 75 F and then let it drop down to 40 F or so when
finished working.) Your argument makes sense, AND the symptoms are exactly
what I saw (a piece of something, which was burning brightly, flew up in a
large arc when the engine exploded. Also, when slowed down, the video shows
a large jet of flame coming out the top of the rocket). Also, on the day I
launched, it was about 25 F outside, and I had just brought the engines
from my house which was at about 65 F.

BTW, yes, I'm now at Cornell and it is cold. However, I conducted the tests
in Moscow, PA (near Scranton), where it is also cold :-)


>As for 30% lower than expected thrust...  I don't think you can accurately
>measure thrust with an altimeter - a 30% error in predicted altitude is
>likely to be mostly due to errors in calculating the rocket's drag, and/or
>weight, and/or air density/etc.  On the other hand, the burn rate of black
>powder varies rather dramatically with temperature, so a cold motor MIGHT
>actually burn rather slower (and longer) than the same motor launched in
>the heat of summer...

Well, my altimeter doesn't just read peak altitude. In fact, it is actually
an acceleration datalogger which I used to obtain altitude. I got the
measured thrust from the acceleration data directly. Actually, the measured
drag was within 1.6% of the predicted value! Cold could have played a
factor here, too, though, because the burn time was 1.83 seconds, instead
of the normal value of 1.62, and the delay period was 5.83 seconds instead
of 5. So, overall, the entire operation of the motor was slowed down.

Thanks for the explanation!

Sean


>BillW
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2001\01\19@035208 by Bob Ammerman

picon face
> >Pressed black powder motors such as the estes c6-5 are known to be
> >sensitive to temperature cycling.  The paper casing and the black powder
> >slug have slight different thermal expansion ratios, and repeated or
> >extreme temperature changes can cause the propellant to separate enough
> >from the casing to allow the flame front to burn between them.  When this
> >happens, the slug ignites all over, overpressurizes the casing, and
> >(usually) flies out the front of the motor/rocket like a roman candle
star.
> >There's a rule of thumb that goes something like "don't launch at a motor
> >temperature more than 30F lower than the max temperature the motor has
been
> >stored at."  (Didn't the original message come from someone up at
Cornell,
> >where I imagine it's very cold at the moment?)

Too bad NASA didn't know this when they launched Challenger :-(

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

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2001\01\19@045254 by Alan B. Pearce

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>Too bad NASA didn't know this when they launched Challenger :-(

I got the impression they did, but over rode the "don't launch" recommendation
from the manufacturers engineers, or maybe it was the manufacturers management
that over rode the engineers and signed it off.

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2001\01\19@105204 by David VanHorn

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At 09:53 AM 1/19/01 +0000, Alan B. Pearce wrote:
> >Too bad NASA didn't know this when they launched Challenger :-(
>
>I got the impression they did, but over rode the "don't launch" recommendation
>from the manufacturers engineers, or maybe it was the manufacturers management
>that over rode the engineers and signed it off.


The shuttle engines are ammonium perchlorate, aluminum, and a binder, which
is what the Aerotech reloads are made from.  Not black powder. The failure
was due to a rubber O-Ring that lost it's flexibility in the low temperatures.

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2001\01\19@105504 by Wynn Rostek

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Sean,

I too have seen about 30% low thrust on the C6-5. I built a rocket of my own
design, calculated the CDA, then wrote software that used the thrust curves
ESTES published in order to simulate the flight. It produced a graph of
altitude vs time for a whole series of engines from the A8-3 through the
C6-7. Armed with several packs of engines the club headed out to the field
and gathered paek altittude data. All of the measurements came in within 5%
of the predictions except the flights using C6-7 which were 30% low.
Everyone wanted to blame the simulation software but I remain unconvinced to
this day.

That was the year that Dave and I went to Huntsville to compete in the
Nationals. While we didn't bring home any trophys, we did have a great time.
ESTES had just changed hands and the new president was attending. Several
competitors (I was not one of them, honest!) rigged ESTES engines to CATO
dramatically. They did this by sawing part way through the paper case and
then jamming the ignitor in with a sliver of wood with a little glue on it.

Just wanted you to know you're not alone.

Wynn Rostek

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2001\01\19@123913 by Wynn Rostek

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> The shuttle engines are ammonium perchlorate, aluminum, and a binder,
which
> is what the Aerotech reloads are made from.  Not black powder. The failure
> was due to a rubber O-Ring that lost it's flexibility in the low
temperatures.
>

The solid rocket boosters are ammonium perchlorate, aluminum and binder. The
failure was due to a combination of poor judgement on managment's parts, and
bad luck.
The O-Ring was not rated for use at that low of a temperature. There was ice
on the shuttle that day. Several people advised against a launch attempt.
Management decided to go anyway. This happens all the time, for a variety of
reasons. It is still going on today. There were at least 4 counts that I
know of in the last 12 months that were recommended against. (Usually for
low probability of acceptable weather.) One was a successful launch. That's
what they pay the guys for, to make the go/no-go call on the launch
attempts. They almost always try, and they frequently get away with it. One
time they didn't. Actually they fail to get away with it all the time, its
just that it usually doesn't result in quite so press coverage.

James Womack Sr's first launch as director of expendable vehicles was a real
disaster. He is the father of a hunting buddy of mine, and he had been
working on the manned side for many years. On the expendable side the rules
a lot different because the payload is owned by a company interested in
getting it in orbit and on line to get the revenue stream going. They have a
whole lot more say in when an attempt is made and when you scrub. This
results in different language being used when you get weather advisories.
The time of launch approached, James asked for a call on the weather, which
was dicy at best, and the weather advisor told him that it was go if James
desired, meaning if the customer wanted to risk it, it was his payload.
James was used to manned weather reports. They launched, the bird blew up
and a grand time was had by all.

Back to the bad luck bit. Few people know it, but the SRB nozzles were
slewwed over, trying to compensate for the side thrust from the leak at the
field joint. (The SRB's are actually sections because you can't cast that
much propellant in one go. The sections are then pinned together and a metal
band hold the pins in place. The result is called a field joint.) They kept
the shuttle on track, leak and all. If that leak had developed on a slightly
different section of the O-Ring, the flame would have jetted out into space,
never would have burned through the external tank, and only a few of us
space workers would have ever known about it. 30 degrees of danger and we
ended up unlucky.

Wynn Rostek

Former Shuttle Worker
CBEP (SRB Power Distribution and Control)

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2001\01\19@140614 by rottosen

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face
For a different and interesting view of how NASA worked during the
Apollo era, read a book titled Failure Is Not An Option by Gene Kranz.
Gene Kranz was a NASA Flight Director for many years. (You may remember
him from the Ron Howard movie "Apollo 13" as the one in the fancy
sweater.)

-- Rich


Wynn Rostek wrote:
{Quote hidden}

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2001\01\19@140828 by David VanHorn

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>
>Back to the bad luck bit. Few people know it, but the SRB nozzles were
>slewwed over, trying to compensate for the side thrust from the leak at the
>field joint. (The SRB's are actually sections because you can't cast that
>much propellant in one go. The sections are then pinned together and a metal
>band hold the pins in place. The result is called a field joint.) They kept
>the shuttle on track, leak and all. If that leak had developed on a slightly
>different section of the O-Ring, the flame would have jetted out into space,
>never would have burned through the external tank, and only a few of us
>space workers would have ever known about it. 30 degrees of danger and we
>ended up unlucky.
>
>Wynn Rostek

Indeed.  BTW: do they grease their O-Rings?
This may be a silly question.. In our hobby rockets, we also use
O-Rings.  We coat them with a silicone or vaseline grease to help prevent
burn-through, and to acheive a better seal. It seems to work pretty well. I
haven't had an engine failure yet :)

I get that their O-rings are for a different purpose, but the end result is
much the same, we're both using them to seal the combustion chamber.  Our
cores are sectioned, but only for postal requirements. Once they're
installed they just sit on top of each other in the casing.

I bet cleanup of those SRBs is fun! I know how much gunk is left in my casings.

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2001\01\20@012942 by Russell McMahon

picon face
I agree with what has been said but I think there's a little more to add.

>The solid rocket boosters are ammonium perchlorate, aluminum and binder.
>The failure was due to a combination of poor judgement on managment's
parts, and
>bad luck.

... and less adequate initial design than could have been achieved at
minimal extra cost.
The weaknesses of the original joint design that were subsequently corrected
were well known in advance.

Of course, hindsight is a marvellous thing. The joint design was such that
it "opened up" under pressure placing greater demands on the O-ring.
The post-Challenger redesign placed a "trapping" ring (or finger in cross
section view) which stopped the joint opening up under pressure.
That said, the decision to launch at below design ambient temperature as a
consequence of "political" pressure was a significant factor.


Really bad ASCII drawing of old and new joint designs.

x is one case half.
y is other case half
O is O=ring.


Old

     yyyyyy
xxxxxxOyyyyy

New
       xxxxx
       xx yyyy
xxxxxxxxOyyyyyy



regards,



               Russell McMahon

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2001\01\21@092002 by Michael P Olson

picon face

       Any PICLISTers that are interested more information on the rocketry
portion of this kind of project are HIGHLY encouraged to visit one or
both of the following websites:

               National Association of Rocketry (N.A.R.)- http://www.nar.org

               Tripoli Rocketry Association (T.R.A.) - http://www.tripoli.org .

       Both sites have information on things like minimum distances from
launcher/spectators to the pad, minimum field sizes for predicted
altitudes, etc. They also have (very similar) Safety Codes. Please read
the safety codes. They're written in slightly cheesey first person style,
but are very good guidelines for building and launching rockets. Plus,
they just might keep you out of trouble with local authorities (not to
mention the laws of physics). You can also find out about local launches,
which will put you in contact with rocket enthusiasts who are the real
source of rocketry information.

       Please note that this is not a solicitation for membership, but an
information referral only.

       Another good site with tons of rocketry links (commercial & private
pages) is Rocketry On-Line - http://www.rocketryonline.com. They have links to
hundreds of sites.

       Mike Olson
       TRA #8366


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